The present invention relates to a hydraulic system for a vehicle, comprising at least one hydraulic, load-carrying assembly which includes at least one movable structural element and at least one hydraulic cylinder for actuating the structural element, said hydraulic system comprising a primary circuit which includes a servo device, a stationary hydraulic pump driven by a motor, a directional valve and said at least one hydraulic cylinder, said directional valve being arranged between the hydraulic pump and the hydraulic cylinder for allowing hydraulic oil to flow to the hydraulic cylinder at a servo signal from the servo device in order to operate said assembly.
The invention also relates to a vehicle comprising such a hydraulic system and a supplementary unit for such a vehicle.
The invention is particularly applicable for a vehicle in the form of a truck provided with forks or a yoke.
In hydraulic systems of the above-mentioned type, the speed at which a hydraulic assembly performs its task is substantially proportional to the speed of the motor of the vehicle. This is true whatever the load applied to the motor may be. In order to achieve the maximum hoisting speed in a lifting assembly provided with forks, e.g. in a fork lift truck, the truck driver has to run up the motor to a maximum even when the forks are not carrying any cargo. Notwithstanding the fact that the motor is at maximum speed, only a fractional part of the motor power is utilised when empty forks are lifted at maximum speed. It is not efficient to utilise the motor in this way, but it results in a high specific consumption of fuel, large exhaust gas emissions and a high sound level. If, on the other hand, priority is given to a low specific consumption of fuel, small exhaust gas emissions and a low sound level, a lower hoisting speed has to be accepted, which influences the hoisting capacity of the truck in a negative sense with respect to the cargo handled per unit of time. However, the industry makes great demands upon productivity and speediness. In many handling situations, the hoisting speed plays an important role, particularly when larger ranges of lift are concerned.
Generally, the above-mentioned is true for all types of hydraulic assemblies of a fork lift truck. However, as a rule, the lifting assembly is the unit which requires the largest quantity of hydraulic oil and, furthermore, for the longest time, which is the reason why the above-mentioned problem normally is largest just in the lifting assembly.
Accordingly, when utilising a hydraulic assembly which performs a hydraulic function in a hydraulic system of the above-mentioned type, there is a general need of optimising the motor speed while maintaining a predetermined maximum speed of the assembly, or alternatively, of maximising the speed of the assembly while taking the load on the assembly into consideration.